Battle Wagon

Sorry if there are too many questions for one thread but they all relate to one project.
So our adventuring party wants to create some battle wagons. However we cant seem to find the price/weight of some of the things we want. First we want to cover our wagons in armour. How much would it cost/weigh to cover a wagon in wood with some murder holes to fire through and a reinforced door. Same question but with metal(iron?).
We also wanted to have a balista trailing each weapon. How much would it cost/weigh to set it up on a rotating fire platform? What proficiencies would we need to design this? Also do you need the siege engineering proficiency to man a balista? And how much would a mercenary with this proficiency cost per month to hire?